De La Plata Estuary Research Articles

Assessing the earthquake hazard around the Río de la Plata estuary (Argentina and Uruguay): Implications for risk assessment

Approximately 18 million people live around the Río de la Plata estuary. Being far away from the zones of major plate boundary-related earthquakes in South America, this region is conventionally regarded as being seismically stable. Nevertheless, during the last 2 centuries of recorded history, there have been several earthquakes of low to moderate magnitude in the region and there is no reason to suppose that it will not continue to be seismically active, albeit at low to moderate levels. This paper examines the evidence and reviews the seismic hazard for the areas bordering the Río de la Plata, and, taking into account the lack of preparedness in terms of construction codes, specifically in Uruguay, concludes that the exposure risk needs review. Until further investigation is carried out, we recommend that consideration be given to raising the seismic hazard rating for the zone around the Río de la Plata estuary to a PGA level of 0.1 g for 10 % probability of exceedance in a 50 year period.

Baseline data of the presence of meso and microplastics in digestive tract of a commercially important teleost fish from the Rio de la Plata Estuary System (Southwest Atlantic Ocean)

In this paper, we analyze the occurrence, abundance, and type of plastic debris (PD) present on the digestive tracts of 65 white croakers (Micropogonias furnieri) catches from Rio de la Plata Estuary System (RLPES). Eighty percent of individuals analyzed had ingested PD, of which 156 were thread-like plastics (FO = 83.9 %) and 30 were fragments (FO = 16.1%). The average number of pieces per digestive tract was 2.83 ± 3.14. Microplastic represented 85% of PD, the majority of these pieces were <2 mm, and the preeminent color was blue. The RLPES is the area of the Southwest Atlantic Ocean that concentrates most of the records of plastic debris on the environment, where the white croaker is the main commercial fish. In addition, M. furnieri constitutes a key prey of several predators in the area. Although preliminary, this is a relevant contribution to the knowledge of marine debris pollution and its impact on the Southwest Atlantic Ocean organisms, particularly at the RLPES.

Water Dynamics in a Coastal Wetland in the “Parque Costero del Sur” Biosphere Reserve, Argentina

Interactions and exchanges between surface water, groundwater, and the atmosphere are some of the most relevant functions that regulate environmental characteristics in wetlands. These functions depend on environmental attributes such as topography, geological-geomorphological setting and, in the case of coastal wetlands, the relationship with tidal flows. The aim of this work is to analyse the water flows in different landscape units of the wetland in the coastal plain of the Río de la Plata estuary within the Parque Costero del Sur biosphere reserve. For this purpose, monthly water balances and groundwater level measurements were compared with daily precipitation and tidal height data from the Río de la Plata estuary. The results show how water dynamics in the wetland responds to different geological-geomorphological and climatic controls. The main inlets and outlets of water, as well as the flow exchanges, vary according to the different landscape units, forming a complex system on which the main ecological features depend.

Long-term assessment of trace elements in franciscana dolphins from the Río de la Plata estuary and adjacent Atlantic waters

The estuary of Río de la Plata, in the eastern coast of South America, is a highly anthropized area that brings a high load of contaminants to the surrounding waters, which may have detrimental effects on the local marine fauna. The franciscana dolphin (Pontoporia blainvillei) is a small cetacean species endemic of the southwestern Atlantic Ocean listed as Vulnerable in the IUCN red list. In this study, we assessed the concentrations of 13 trace elements in bone samples from 100 franciscana dolphins that were found stranded dead or incidentally bycaught in the Río de la Plata and adjacent coast between 1953 and 2015. Elements were, in decreasing order of mean concentrations: Zn > Sr > Fe > Al > Mn > Cu > Pb > Cr > Ni > As > Hg > Cd > Se. The concentrations of Al, Cr and Fe were slightly higher in females than in males. The concentrations of As, Ni, and Pb significantly decreased with body length. Throughout the study period, the concentrations of Al, Cr, Cu, Fe, Mn and Ni significantly increased, while the concentrations of As, Pb and Sr significantly decreased. The increasing trends may be due to increased inputs from river discharges, the leather industry and petroleum refineries, while the decrease in Pb may be due to the ban in the use of this element as an additive in gasoline and as component of car batteries. This investigation supports the validity of analysing trace element in bone, a tissue available in scientific collections and museums, to retrospectively examine variation over long temporal scales and thus assess long-term trends in pollution.

Conserving the Paraguay-Paraná Fluvial Corridor in the XXI Century: Conflicts, Threats, and Challenges

The Paraguay-Paraná river system represents a unique, free-flowing corridor that extends about 3500 km southwards from the Pantanal to the Rio de la Plata estuary, crossing four countries. The absence of fragmentation along the main channels and its still well-connected floodplains have preserved longitudinal and lateral functional and structural connectivity, defining critical ecological gradients for the biota and species life cycles. The lack of dams represents a noticeable feature that benefits migratory species supporting small-scale fisheries providing food security and the conservation of other ecosystem services with associated livelihoods. The ecological integrity of this corridor could be severely affected by the potential expansion of the Hidrovía Paraguay-Paraná for improving commercial trade, the foreseen installation of new dams in the Paraná basin, in addition to other current impacts and threats. Conservation of the corridor for societal benefits, involving the sustainable development of activities associated with the use of the fluvial territory, requires maintaining the natural ecological process that sustains livelihoods and biodiversity. This calls for innovative strategies encompassing water governance process, social and environmental information related to expected impacts, a better understanding of synergies between processes, and foremost an interdisciplinary approach to design and apply integrative and multi-scale management policies.

Temporal patterns in the abundance, type and composition of microplastics on the coast of the Río de la Plata estuary

The objective was to establish possible temporal patterns in the microplastics (MPs) abundance in the water and in the intertidal sediment in an urbanized location of the Río de la Plata estuary (Argentina), in relation to environmental factors. The site was sampled monthly for a year (February 2019–January 2020). The presence of MPs was recorded, being more abundant in February in the water (110 MPs m−3) and in April in the sediment (613 MPs m−2). The types of MPs found were fibers, fragments, pellets, film and foam; while the polymers identified were polyethylene (PE) and polypropylene (PP). Regarding the environmental variables, the predominance of wind from the NE direction was related to a greater accumulation of MPs in the sediment, while wind from the NO direction was associated with a lower abundance of MPs in the water. Also, MPs abundance was negatively related to the larger sizes of sediment particles.

Metal geochemistry in suspended and bed sediments in the eutrophic lowland Salado River basin (Buenos Aires, Argentina)

The Salado River is a lowland river that drains a vast and productive region of central Argentina into the Río de la Plata estuary. To evaluate water quality and the geochemistry of major and trace metals (Fe, Mn, Zn, Cu, Pb, Cr and Ni), water and sediments samples were collected along 500 km of the Salado River basin (n = 21). Waters were highly alkaline (pH: 8.8 ± 0.4), saline (5.2 ± 1.8 mS/cm), turbid (467 ± 237 NTU) and eutrophic (chlorophyll a: 187 ± 119 µg/l) with a decreasing gradient towards the river mouth reflecting the contribution of alkaline ground waters and eutrophication at the river head. Suspended particles (70–595 mg/l) were enriched in total organic carbon, nitrogen (5–7 times) and metals (1–4 times) relative to bed sediments, but metal concentrations were 2–10 times below the world river’s average. Consistently, metal concentrations in bed sediments were lower than sediment quality guidelines with an increasing downriver trend following the enrichment of clays. The enrichment factors (EF) and geoaccumulation index (Igeo), calculated with local background metal levels from the deepest layers of a dated sediment core, indicate a general prevalence of natural metal sources (EF < 1.5) with unpolluted sediments (Igeo ≤ 0), excepted for the Chivilcoy stream, which showed a significant enrichment of Zn, Cu and Pb (EF = 2–12; 0 ≤ Igeo ≤ 1) constituting a clear pollution hot spot in the basin.

Biogeochemical multi‐tag approach reveals the habitat use of a large‐scale migratory fish through a fluvio‐estuarine system

AbstractThis study assesses the use of stable isotopes (δ13C and δ15N) and trace elements in gills (G), liver (L), and muscle (M) of the most commercially important migratory fish from the La Plata Basin, Prochilodus lineatus, as habitat proxies. With this purpose, fish and sediment (S) samples from the Paraná River and Río de la Plata Estuary were analyzed. Results showed differences in the stable isotopic ratios in fish tissue and sediments between the river and the estuary. In particular, δ15N values of the estuary samples were significantly lower (G = 7.38‰; L = 6.60‰; M = 8.61‰ and S = 4.36‰) than those from the river (G = 11.30–11.53‰; L = 11.03–12.35‰; M = 11.27–11.31‰, and S = 6.78–7.72‰), which could denote a shift in the isotopic baseline between these environments. Quadratic discriminant analysis effectively discriminates between sampling sites with high classification rates (L = 97.5%; G = 90%, and M = 82.5%), suggesting that combining stable isotopes and trace elements is useful to discriminate between fish populations. The liver was the most suitable tissue as a habitat indicator, using mainly δ15N, δ13C, Cr, and Hg as natural markers, and capable of discriminating between sites with a relatively high geographic resolution. Due to the spatial isotopic heterogeneity found in the La Plata Basin, these natural markers could be useful for studying other aquatic organisms' habitat use.

Río de la Plata voyagers: Deciphering the migration ecology of a vulnerable marine catfish (Genidens barbus) in a large subtropical river (lower Uruguay River)

Abstract Genidens barbus is a vulnerable marine migrant catfish with low fecundity, a complex life cycle (i.e. mouth breeding), and is the target of industrial and artisanal fisheries of several countries. This species regularly migrates from marine to freshwater environments of the south‐western Atlantic. The aim of this work was to delve deeper into the migration ecology of G. barbus, characterizing both its timing and potential environmental drivers. Furthermore, aspects of the population structure and reproduction of migrants in the lower Uruguay River were studied and the presence of juveniles in the adjacent estuarine recruitment area was evaluated. Data from 11 years (2008–2018) of records of adult G. barbus captured by artisanal fisheries were used alongside relevant environmental variables that were recorded monthly. Reproductive biology (i.e. sex ratio, gonadosomatic index, fecundity, and oocyte size) was analysed for a period of 3 years (2016–2018). The juvenile abundance in the Río de la Plata estuary was evaluated seasonally. A total of 935 adult individuals of G. barbus were captured, representing a total biomass of 3,123 kg. The migration timing was from early spring to early summer. The abundance of migrants strongly increased with river discharge, suggesting that this variable regulates the upriver migration. Furthermore, pre‐ and post‐spawn females and males displaying mouth breeding were recorded during the study period, confirming G. barbus reproduction in the lower Uruguay River. The results obtained suggest that G. barbus ascend to spawn in the freshwater environments upstream from the mouth of the Uruguay River. Then, adult males incubate and carry the embryos downstream, releasing juveniles in the Río de la Plata estuary. This relevant information will help with the implementation of effective management polices (e.g. fishing restrictions during the reproductive period) for the presently unregulated fishery of this vulnerable species in the lower Uruguay River.

Suspended particulate matter is a key factor shaping the spatial arrangement of Micropogonias furnieri (sciaenidae) population in a Southwestern Atlantic temperate estuary

The relative influence of suspended particulate matter (SPM), sea surface temperature, depth, and type of bottom sediment in the spatial distribution of the Micropogonias furnieri (Sciaenidae) population structure was studied in the Río de la Plata estuary. Eight size-assemblages were defined (Cluster Analysis) showing a significant association with environmental variables. The application of the Boosted Regression Trees model for presence/absence and relative abundance showed that the SPM (46.24% and 34.24%, respectively), followed by the depth (31.47% and 33.50%, respectively) were the most important environmental variable in predicting the spatial distribution. Results showed a population segregation pattern. Juvenile and small adolescent fish increased their probability of occurrence and their relative abundance with increments of SPM and decrease of depth. The large adolescent and adult fish substantially increased their occurrence and relative abundance with the decreasing of SPM concentrations at low values. This study quantified and showed the high importance of the SPM in the spatial distribution pattern of the population structure of an estuarine species. In this context, it proposed that the SPM in the Río de la Plata could be used as an indicator of habitat availability of juveniles/adults within an ecosystem approach for fisheries management.

The origin of gley colors in hydromorphic vertisols: the study case of the coastal plain of the Río de la Plata estuary

This work aims to contribute to the interpretation of soil colors; mainly to identify the pigments that generate the greenish colors in hydromorphic conditions (gley colors) in vertisols of the coastal plain of the Rio de la Plata estuary. These colors are of cold hues, generally greenish (olive-gray), and occur in poorly drained soils with prolonged water accumulation. For this purpose, samples of Bssg horizons of hydromorphic vertisols from the coastal plain of the Rio de la Plata estuary were analyzed using routine and chemical analyses, Mossbauer spectroscopy, determination of rocks magnetic parameters, X-ray diffraction, thermogravimetric and differential thermal analysis and petrographic-electronic microscopy. The pigments associated with the gley colors of these soils are mainly due to ferric iron content, corresponding to minerals such as ferric iron-rich smectites (nontronite/Fe-rich beidellite) and oxy-hydroxides like goethite, which colors range from green to yellow. These minerals, combined with gray or black components, e.g., manganese compounds and/or organic matter, contribute to the generation of the usual gley colors in the analyzed soils of the Pampean plain. The obtained data allows us to state that it is a mistake to assing the olive color in hydromorphic soils to the presence of ferrous iron compounds, as it is traditionally done. The oxidizing condition of iron is dominant in all the analyzed samples, integrating the composition of ferric iron-rich clay minerals and oxy-hydroxides (goethite).

Distribution of freshwater fish from the Southern Neotropics reveals three new areas of endemism and show diffuse limits among freshwater ecoregions

Abstract Ecoregions and areas of endemism are central concepts in biogeography. Based on collection records and the Endemic Analyses implemented with NDM/VNDM method we analyzed fish areas of endemism in the junction of three freshwater ecoregions related to the Rio de la Plata estuary (Lower Parana, Lower Uruguay, Laguna dos Patos) in Southern Neotropical region. Using two grid cell sizes, results obtained showed the same general patterns. Areas of endemism recovered were mainly associated either to Lower Uruguay or to Laguna dos Patos. In both ecoregions nested areas of endemism were identified within larger patterns of endemism. Noteworthy, one area recovered occurred across Lower Uruguay and Laguna dos Patos limits. Our results also suggest a revision of the Lower Uruguay and Lower Parana ecoregion limits, and highlight the relevance of the Rio de la Plata estuary as a barrier and corridor for freshwater fishes in the area.

Study of the plastisphere: biofilm development and presence of faecal indicator bacteria on microplastics from the Río de la Plata estuary

Microplastics (MPs) are considered emerging pollutants and often enter aquatic ecosystems carried by currents and tides until they accumulate on the shorelines. In many cases they may be colonized by diverse microorganisms forming a community called plastisphere, which can even act as a reservoir for pathogenic microorganisms. This study carried out in the Río de la Plata estuary (southern coastal fringe, Argentina) focused on two main objectives, the analysis of the biofilm colonizing MPs under laboratory conditions, and the detection of bacteria indicating faecal contamination (Escherichia coli and Enterococci), in MPs from the intertidal sediment at coastal sites with different land uses, in the freshwater sector of the Río de la Plata estuary. The colonization experiment was carried out in the laboratory with water from the estuary for a period of 35 days (residence time of the water in the freshwater sector of the estuary). The results revealed a remarkable development and diversity of biofilm organisms from the second week of colonization on, covering the surface of the microplastic and thus masking this pollutant. On the other hand, the presence of faecal indicator bacteria in the MPs of the intertidal sediment was confirmed in all the studied sites, being proportionally higher on MPs found in areas influenced by sewage discharges.

Ventajas y límites de la variabilidad morfofisiológica para el establecimiento, la colonización y la expansión del bivalvo invasor Limnoperna fortunei en Sudamérica

Phenotypic variability, both based on phenotypic plasticity and genetic diversity, can be a main factor affecting the invasion process of alien species. Limnoperna fortunei (Dunker 1857) was introduced in South America around 1990, in the Río de la Plata estuary, Argentina, from where it spread swiftly to cover most of the basin and other smaller watersheds. However, some environments have not been colonized and, presumably, the survival of this species was affected by different environmental factors such as suspended solids, salinity, and pollution. L. fortunei showed high genetic and morphological variability, showing a distribution strongly affected by human activities and its dispersal vectors. The degree of phenotypic variability would be playing a main role in its establishment, or not, in the different environments. Morphological and metabolic variations, including changes in the growth rate, development of internal organs or changes in the rate of filtration and respiration in border areas of its distribution or under extreme environmental conditions, would allow to explain, together with other variables such as the presence of dispersal vectors, the current distribution pattern.

Metamorphosis of whitemouth croaker Micropogonias furnieri (Pisces, Sciaenidae)

Fish metamorphosis is an important ontogenetic process with a key role on early stages survival and on successful recruitment to adult populations. The whitemouth croaker (Micropogonias furnieri) is an important commercial resource for the coastal fisheries of Argentina and Uruguay. Metamorphosis, using morphometric and morphological analyses during larval development was studied. Changes in morpho-meristic characters before and after metamorphosis were employed to determine the length interval of this transition by employing Principal Component Analysis. Individuals (n=430) from 4 to 41 mm standard length (SL) were collected in the Río de la Plata estuary (35.45º S, 56.35º W) in March 2006. Length ranges of individual´s developmental stages were associated with the presence of key morphological characters. During early life stages, M. furnieri changes from a big-headed, robust shape larva to a slender and more elongated body form. Most of the morphometric variables showed an inflexion point at 15.2 mm SL, with a 95% confidence interval of 14.0 – 16.4 mm. The anterior part of the body grows faster during early stages, probably related to an intense feeding activity strategy. The completion of pectoral fin rays and the onset of squamation determine the beginning of metamorphosis at 11-12 mm SL. At around 18 mm SL, squamation ends, first barbels develop and the sagittae otolith primordium is closed. The length-at-metamorphosis for M. furnieri was established between 9 to 18 mm SL, since all developmental characters studied highly overlapped at that interval. All those processes are indicative of the beginning of the juvenile period associated to the settlement and the start of a bottom-oriented life-style.

Accumulation of human pharmaceuticals and activity of biotransformation enzymes in fish from two areas of the lower Rio de la Plata Basin

The accumulation of four human pharmaceuticals active compounds (HPhAC) in the muscle of four fish species of the Rio de la Plata Basin were assessed regarding the compound logP and pKa, and fish trophic levels. For Prochilodus lineatus, accumulation was compared to hepatic biotransformation enzymes and fish from two sampling areas with different urbanization degree. Species were the detritivore Prochilodus lineatus, the omnivores Megaleporinus obtusidens and Pimelodus maculatus, and the piscivorous Salminus brasiliensis. Sampling areas were the Inner Rio de la Plata Estuary (RLP), in front of the Buenos Aires Metropolitan Area, and at the Lower Uruguay River (URU), a relatively unpopulated area. Carbamazepine, atenolol, enalapril, and sildenafil concentrations in fish muscle were analyzed by HPLC-MS. EROD, BROD, and GST activities were measured in P. lineatus liver. Average (maximum) concentrations and detection frequency were: Atenolol 24.4 (69.4) μg kg−1, 60%; carbamazepine 5.5 (45.8) μg kg−1, 19%; enalapril 7.0 (56.9) μg kg−1, 28%; sildenafil 17.1 (71.6) μg kg−1, 56%. Enalapril and sildenafil detection in fish was first-time reported. Atenolol and carbamazepine concentrations were positively correlated. No correlation was observed between HPhAC accumulation and logP or pKa. A potential biomagnification trend was observed for atenolol, showing higher accumulation in S. brasiliensis. HPhACs accumulation was higher for P. lineatus collected at URU, but GST and BROD were significantly higher for individuals sampled at RLP and were positively correlated. A significant negative correlation was observed for enalapril and BROD. The study contributes to the knowledge of the accumulation of HPhACs in fish.

Development and evaluation of an ensemble forecast/hindcast system for storm surges in the Río de la Plata Estuary

AbstractThe development and evaluation of ensemble hindcasting and forecasting systems (EHS and EFS, respectively) for storm surges in the Río de la Plata Estuary (RdP) is presented. The models were forced by atmospheric sea level pressure and 10 m winds. The ensemble forcing for the EHS was generated by temporal‐spatial shifting of the operational global control ERA5 reanalysis provided by the European Centre of Medium‐Range Weather Forecast (ECMWF), because the ERA5 associated ensemble shows too little dispersion in this area. EFS was instead based on a lead time of 4 days and forced with the 50‐member high‐resolution ensemble prediction system of the ECMWF. EHS was evaluated over a long period (the 2000–2010 decade), whereas EFS was evaluated for the ten most extreme surges that occurred during that period: five positive (which caused coastal flooding) and five negative (which affected navigation and drinking water supply) events. Based on traditional statistics (area under the ROC curve and Brier scores), both systems were assessed from a probabilistic point of view. Results show that both EHS and EFS can incorporate more than 90% of the observations in the uncertainty range. They also showed good skill in hindcasting and forecasting surges, particularly extreme events, EHS being about 20% better than the control model and EFS up to 55% better, in agreement with state‐of‐the‐art models developed for other parts of the world. Results also showed that EFS can predict most of the surge peaks with 95% confidence, with a range of uncertainty of about ±0.90 m and ±9 hr. Therefore, results encourage the implementation of EHS and EFS as useful and robust tools for future climate studies, decision makers and the general public, to improve the quality of risk management decisions by quantifying forecast uncertainty.

Nonlinear Interaction Between the Tide and the Storm Surge with the Current due to the River Flow in the Río de la Plata

The Rio de la Plata estuary (RdP) is characterized by the large flow of its tributary rivers (Q), with an average of 22,000 m3 s− 1 and an interannual variability range from 8000 to 90,000 m3 s− 1. In this work, the hypothesis that the current due to that flow (CDR) interacts nonlinearly with both the tides and storm surges is evaluated utilizing water level observations and numerical simulations. Two tide gauge time series gathered at the freshwater tidal zone (FTZ) of the RdP are analyzed with the novel surrogate analysis. The analysis is applied for periods of high, medium and low Q. Results show that both interactions occur at the upper half of the FTZ and increase with Q. Harmonic analyses support the surrogate analysis’ conclusions and show that tide-CDR interaction redistributes the energy among tidal harmonics, increasing asymmetry. Numerical simulations confirm that (i) both interactions maximize at the upper half of the FTZ and decrease downstream; and (ii) they are modulated by Q; a rise of about 14,000 m3 s− 1 (interquartile range) can produce an intensification of 50% and 100% of the amplitudes of the tide-CDR and surge-CDR interactions, respectively; and (iii) both interactions introduce asymmetries in the water level, with faster rises and slower falls; (iv) the quadratic bottom friction is the main source of both interactions; (v) tide-CDR interaction represents 12% of the water level associated with the tide, whereas surge-CDR interaction accounts for 5% of the surge peak; and (vi) the interactions are significant in the upper FTZ because there, the magnitude of the currents associated with the tide and the surge are comparable to CDR; downstream, the channel widens and CDR decreases.

The contemporary strain rate field in Uruguay and surrounding region – vertical velocities

This study supplements analyses of the horizontal strain rate fields in the region in and around Uruguay presented in Baxter and Smith (2020). Whilst the original study did not consider the vertical component, because of high levels of noise and discontinuities in the raw data, these effects have been minimized and appropriate analyses are presented with consideration of the uncertainties. The results show that, similarly to the horizontal strain rate field, the vertical velocity field in the region appears to have been influenced by two subduction earthquakes that occurred in Chile. The patterns of uplift and subsidence revealed by these analyses suggest influences from the underlying structure of the Rio de la Plata estuary, and perhaps the Mesopotamian region in Argentina, that may relate to enhanced seismicity in these specific areas.

Parasitism and fitness of invaders: oligochaete Chaetogaster limnaei produces gill damage and increased respiration rates in freshwater Asian clams

Biological interactions between invasive and resident species are perceived as a key factor mediating impacts in invaded ecosystems, although symbiotic interactions have received relatively little attention. To evaluate the association between the invasive clam Corbicula fluminea and its oligochaete symbiont Chaetogaster limnaei, we conducted laboratory assessments of prevalence and intensity of infection, external and internal macroscopic alterations, and histological analyses of the gills of uninfected and infected clams collected from the shores of the Rio de la Plata estuary in Argentina. To evaluate potential ramifications of physical alterations on physiological traits and host fitness, we conducted respiration experiments on the same clams. Worm prevalence was higher in smaller than large clams. While no macroscopical alterations were detected, several histopathological changes were observed in the gills of infected clams. Infected clams had significantly higher metabolic rates than uninfected conspecifics and smaller clams had higher metabolic rates than larger conspecifics. Increased host metabolic rates, even at very low infections intensities, were likely a compensatory response to the stress caused by gill damaged inflicted by the oligochaetes. The relationship between C. limnaei and invasive C. fluminea in Rio de la Plata best fits as parasitism.